The three free sites where you can sell e-books are Amazon, Nook, and Kobo. To sell e-books on Apple’s iBookstore, you need to buy an ISBN, which immediately cuts into your potential profits so you should stick with Amazon, Nook, and Kobo initially. Although Amazon has the largest presence, you may find that your e-books may sell better on Nook or Kobo.
In my own experience, I sell the bulk of my e-books through Amazon. However, I wrote a humorous cat book called “How to Live with a Cat (When You Really Don’t Want To),” which strangely sells nearly as many copies on Nook as on Amazon. Even though Nook has a far smaller audience than Amazon, this particular e-book seems to sell equally or even slightly better on Nook than on Amazon.
Another curiosity is my children’s math book called “Math for the Zombie Apocalypse,” which teaches math tricks as if the reader were a survivor of a zombie apocalypse. This e-book seems to have sold equally well on Amazon, Nook, and iBookstore.
On the other hand, the bulk of my screenwriting book “The 15-Minute Movie Method” sells mostly on Amazon with an occasional sale through Nook and iBookstore.
So far I’ve had zero success selling anything on Kobo.
What does this all mean? It’s hard to reach any definite conclusion, but in general it means that your e-book could sell better on a smaller e-book retailer like Nook than on a much larger one like Amazon. Who knows why, but that’s just a fact. Perhaps your particular audience just happens to use one e-book retailer more than another.
That’s why you should promote your e-book through multiple retailers. Amazon is the biggest so chances are you’ll have more success through Amazon than through any other retailer. Nook is the second biggest so you might have some success selling through Nook. iBookstore requires that you purchase an ISBN for each e-book you want to sell, so that’s an expense upfront. If you have a particularly popular e-book, then you can consider selling it through iBookstore too.
As far as Kobo goes, maybe others have better success with it than me. Since it’s free to submit e-books to Kobo, you have nothing to lose by trying to sell your e-books on Kobo. Just make sure you also sell them on Amazon and Nook as well.
The e-book market can be fairly predictable, but it will surprise you. In general, expect to make more sales through Amazon simply because of Amazon’s vast global audience. Just don’t be surprised if one of your e-books starts selling better somewhere else. As long as you keep publishing related e-books that satisfies your audience, you can be assured of continuously selling e-books no matter which retailer happens to be more popular at any given time.